The invention relates to a method for detecting a gap between strands in fiber fabric and a device for its implementation.
Fiber fabrics are used in the manufacture of composite material parts.
Fiber fabric consists of several layers of fibers stacked on top of one another and sewn together. Each layer of fibers comprises a plurality of juxtaposed and parallel strands. As an order of magnitude, a strand has a width of approximately 3 mm and thickness of approximately 0.25 mm. Finally, each strand comprises a plurality of juxtaposed fibers.
Generally speaking, fiber fabric is in the form of a long strip having a width in the order of 1400 mm and a length of several tens of meters, for example. For the remainder of the description, the longitudinal direction corresponds to the length of the strip. This longitudinal direction is parallel to the X axis in FIGS. 1, 2A and 2B.
According to an configuration illustrated in FIG. 1, a fiber fabric 10 is said to be multiaxial and includes several layers, each layer having its own orientation, e.g. a first layer 12 with fibers oriented at 90° which are arranged perpendicularly to the longitudinal direction, a second layer 14 with fibers oriented at +45° which form an angle of +45° with the longitudinal direction, a third layer 16 with fibers oriented at 0°, which are arranged parallel to the longitudinal direction and a fourth layer 18 with fibers oriented at −45° which form an angle of −45° with the longitudinal direction.
Each layer of fibers must have a homogeneous surface density of fibers regardless of the size of the reference surface area so as to obtain a composite material part that is in compliance with the mechanical requirements. For example, the fiber surface density of a layer of carbon fibers is 270 g/m2 for a thickness of approximately 0.25 mm.
As illustrated in FIG. 2A, to obtain a homogeneous fiber surface density, the strands 20 of the layer are juxtaposed, rectilinear and parallel to one another. Thus, the spacing between two adjacent strands must be constant over their entire length.
As illustrated in FIG. 2B, a layer of fibers comprises a defect 22, with two adjacent strands 24 and 24′ being spread apart over a length L with a maximum distance d. The strands overlap on either side of the defect 22. Thus, this fiber layer has a heterogeneous fiber surface density with above-average surface density in the areas where the fibers overlap and a below-average fiber surface density between the spread fibers 24 and 24′.
A composite part containing such fabric will contain a defect. If this defect is located near a drill hole, the overlapping fibers may be severed during the drilling operation, while some of them would not have been cut had they not been overlapping. The cutting of additional fibers leads to a weakness in the composite material part.
To limit the risk of weakening composite material parts, the gap between two adjacent strands of the same layer must remain below a given threshold in the order of 2 mm.
According to a procedure, the process for manufacturing multiaxial fabric comprises the following steps.
For the fiber layers oriented at 90°, +45°, and −45°, for each layer, the strands are cut according to the width of the multiaxial fabric, then they are arranged by an automatic manipulator on a production line conveyor which advances the fabric. Following this removal step, it is possible to correct any gaps between the strands. The ends of the strands of layers at 90°, +45° and −45° are then maintained by clamps arranged on either side of the conveyor. These clamps exert a slight traction on the strands and hold them in position until the layers are sewn together at a stitching station. For this purpose, the movement of the clamps is synchronized with that of the conveyor.
As the strands of the layers at 90°, +45° and −45° are maintained under tension up to the stitching station, the risk of a spacing greater than 2 mm between the strands is negligible.
It is a different situation for the strands of layers with fibers oriented at 0°.
The length of these strands, which are oriented in the longitudinal direction, is equal to that of the strip of the multiaxial fabric which can reach several hundred meters. Each strand oriented at 0° is maintained straight solely by the tension applied to it between two points, one in the zone where the strand enters the production line and the other at the stitching station.
Given the distance separating these two points (in the order of 20 to 30 m) and the slight amount of tension exerted on the strands, it is possible that, as the layers move toward the stitching station, that this tension drops on one of the strands oriented at 0° and no longer maintains its tension. As the strand is initially wound, it tends to lose its straightness. Consequently, its spacing from the adjacent strand may exceed 2 mm. When the tension returns, the latter is too weak to again tighten the previously slack strand in a rectilinear manner. Thus, owing to existing friction between said strand and the strands of the adjacent layers, for example, the slack strand maintains its position and its spacing with an adjacent strand of the same layer, which can be greater than 2 mm.
Due to the nature of the manufacturing process, the risks of spacing in excess of 2 mm between two adjacent strands of a layer at 0° are non-zero.
Consequently, a defect such as this in a multiaxial fabric must be detected before it is used in a process to manufacture a composite material part.
A visual inspection is sufficient when the layer at 0° is visible. However, this layer at 0° is generally inserted between other layers and is not visible.